Work machines may sometimes include one or more caster wheels which are carried by a machine frame and free to rotate about a generally vertical axis 360°. The caster wheel assembly typically includes a shaft defining an axis of rotation, a fork rigidly attached to the bottom end of the shaft, and a caster wheel coupled with the distal ends of the fork. Examples of such work machines include windrowers, combines, lawn mowers, etc.
Self-propelled windrowers are typically driven through a dual-path hydrostatic system. Speed changes are made by adjusting the speed of both drive (front) wheels simultaneously. Direction changes are made by adjusting the relative speed of the drive wheels. The rear wheels of the machine are castered to allow the machine to pivot during direction changes.
When direction changes are made, hydraulic pressure builds in one drive wheel circuit to increase speed and is reduced in the other drive wheel circuit to lower the speed. This relative pressure difference prevails until the inertia of the machine and the inherent turn resistance of the casters is overcome. If the turn resistance is high enough to produce a noticeable delay in the reaction to the steering wheel input, control of the machine can be difficult.
Turn resistance of the caster wheels results from friction in the pivot of the caster assembly and friction between the castered wheels and the ground. Reaction delay can be particularly pronounced if the machine is operated without the cutting platform because the added weight on the casters results in increased turn resistance. Low inflation pressures (e.g., 14 psi) are often specified in the castered tires to improve ride quality. This further increases turn resistance if the machine is operated with the platform removed.
Steering characteristics are dependent on such things as steering linkages, hydrostatic pump reaction time, the machine's turning inertia, and caster turn resistance. There is a tendency for a steering input to have a slow reaction (understeer) at initiation, then a tendency to keep turning (oversteer) when the input is stopped or reversed. Because of this, control of the machine can be difficult, particularly at higher speeds. Windrowers typically have a maximum speed in transport in the 15 mph range. Transport speeds up to 25 miles per hour (mph) would be an advantage in the market. This requires better machine controllability at higher speeds without sacrificing the agility of the current system (spin steer) at lower speeds.
It is known to provide a turn assist arrangement for the rear caster wheels on a work machine. To that end, reference is hereby made to U.S. patent application Ser. No. 11/957,800, entitled “STEERING ASSIST FOR A REAR CASTER WHEEL ON A WORK MACHINE”, filed on Dec. 17, 2007, which is assigned to the assignee of the present invention and incorporated herein by reference (hereinafter the '800 application). With a turn assist arrangement as disclosed in the '800 application, a steering assist valve which controls a flow of hydraulic oil to the pair of steering assist cylinders is coupled directly to the steering wheel column. Typically the operator's cab is positioned in close proximity overlying the frame of the work machine, and the space available for the steering assist valve can be very limited.
What is needed in the art is a steering assist arrangement for a work machine having rear caster wheels with improved versatility to accommodate tight space restrictions.